The invention relates to an improved apparatus for trimming the uneven surface of an end of aluminum can bodies after the can bodies have been drawn and ironed. Aluminum cans are manufactured from flat sheets of aluminum, which are initially formed into small cups that are much shorter and have a larger diameter than typical finished aluminum cans, such as twelve ounce beverage cans available in the retail market today. The cup is further processed by drawing and ironing, to elongate it into a full can body and reduce the diameter. This process produces an irregular surface of uneven height around the circumference of the open end of the can body. This irregular open end can create problems in further processing of the can. For instance, the can may be rejected at a later step of processing called “the necker” for being out of specification, or if it passes through the necker and is filled and sealed with a lid, the seal may be defective due to the irregular surface at the open end of the can, which may lead to failure of the can.
One way of reducing the likelihood of the can being rejected at “the necker” or resulting in failure due to an inadequate seal caused by this irregular surface, is to trim the open end of the can prior to further processing. Typically, a rotary can trimmer is incorporated into the production line downline from the can body maker equipment. After exiting the body maker, can bodies are typically loaded into an infeed chute that places the can bodies into a star wheel, which feeds the cans to the trimmer. As the cans rotate around the star wheel, they reach the trimmer cartridge one at a time. Prior art trimmer cartridges such as the rotary can trimmer available from Belvac Production Machinery typically include a pair of substantially parallel rotating blades or knives that open and close to allow the can body to be placed in position for trimming and to be removed after trimming is complete. Prior art blades are spaced apart by gap of about 0.0010±0.0002 inches, in a direction perpendicular to the opening and closing direction. The blades typically overlap in the vertical direction when in a closed position to shear the irregular surface from the top of the can body.
Once a can body reaches the trimmer cartridge of the prior art, it is typically held by a vacuum in a horizontal position or a position perpendicular to the opening and closing direction of the blades with the open end of the can toward the blades, placed so that one blade is inside the can body and the other blade is outside the can body. With the blades closed, the can body is then rotated to cause the blades to trim the entire circumference of the can. After trimming is complete, the can body is removed from the trimmer cartridge and discharged through a chute for further processing, such as cleaning, printing, and necking. The trimmed waste piece is discarded away from the trimmer.
Prior art trimmer cartridges have several problematic features. One problem caused by the size of the gap between the blades in the prior art is that the can body may be left with burrs or a jagged edge, particularly at the stop/start point in trimming. A burr or jagged edge at the top of the can body may result in rejection of the can body at downline equipment or result in a defective seal, which are the types of problems that trimming is designed to overcome. Also, the gap between the blades in prior art trimmers is usually only adjustable from inside the housing, requiring the cartridge to be removed to make an adjustment in the gap. The gap is also typically set using shims, which are more likely to be inaccurate or cause fluctuations in the gap during operation.
Additionally, the housing of prior art trimmers, such as the Belvac trimmer, is usually not sealed to hold in lubricant and keep out contaminants. Aluminum shavings and body coolant are possible contaminants that can enter the equipment in a non-sealed system, resulting in more down time for cleaning and repair.
Another drawback of prior art trimmers, such as the Belvac trimmer, is found in the tang, which connects the trimmer cartridge housing to the trimmer equipment and provides rotational power to the trimmer cartridge. Tangs are typically square or octagonal, designed to fit in a corresponding opening in the trimmer machine to be rotated and in turn to rotate one of the trimmer cartridge shafts. The rotation on the tang can cause the edges of the tang to be worn or rounded, requiring the tang to be replaced. Usually, a prior art tang is an integral part of the trimmer housing or an integral part of a blade shaft located within the housing. When it is necessary to replace or perform maintenance on the tang, prior art trimmer cartridges have to be opened, and typically the entire tang and shaft have to be replaced. This increases the downtime required to make the replacement or perform the maintenance, which ultimately increases the expense involved.
Still another problematic feature of prior art trimmer cartridges is the use of helical gears. Although helical gears have their advantages, they also have their disadvantages. The primary disadvantage associated with helical gears is that the angle of their teeth creates a thrust load on the shafts on which the gears are mounted, which must be carried by bearings. A further drawback of prior art. trimmer cartridges relates to the shafts on which the blades are mounted. The use of helical gears, which create thrust on the shafts that rotate the blades is another problematic feature of prior art trimmers. Typically, these shafts are not the same size and consequently are of unequal mass. Temperature changes in the trimmer cartridge will cause unequal thermal variations in the shafts because of the mass differential. These thermal variations create fluctuations in the spacing of the blades, which can result in inconsistent or inadequate trimming of the cans.